150 ALASKA GLACIERS 



groove and are there gathered into trunks which follow 

 the groove lengthwise till escape is found through some 

 break in the ridge. The surfaces of trunks and tributaries 

 are mutually adjusted where they join, but it is easy to 

 imagine that the progressive erosion of their beds is re- 

 lated to the volumes of the glaciers, and that the greater 

 ice streams have the deeper channels. It is interesting 

 to note in this connection that the bottom of this trough 

 is below present tide-level at least in part in spite of 

 the fact that there has here been a post-Pleistocene eleva- 

 tion of land. Figure 74 shows a portion of the trough 

 where it is occupied partly by glaciers and partly by an 

 arm of the sea. The observer stands on the foothill ridge. 

 Lituya Bay has the form of a letter T, the cross-bar ap- 

 pearing in the view, and the shaft running through the foot- 

 hills at the right. The large glaciers in the distance and 

 foreground reach the longitudinal groove from deep 

 mountain gorges. A small glacier, whose end barely 

 touches the water, cascades over a sill that may be 800 

 feet above tide. A hanging glacier at 2,000 feet or more 

 sends a tongue down a shallow groove in the steep wall 

 ,of the fiord. And, at the extreme left, a pocket glacier 

 occupies a hanging valley at an altitude of 3,000 feet. 



The depth of the glacial excavation in the passages by 

 Pitt and Princess Royal islands was probably about as 

 great as in the passage by Vancouver Island. The scale 

 of the topography is not far different, the evidence from 

 hanging valleys is equally cogent, and the case is strength- 

 ened by the presumption in favor of pre-glacial summit 

 levels. Even with a low base-level it is not at all prob- 

 able that pre-Pleistocene streams carried the erosion of 

 any considerable part of these troughs below present sea- 

 level, and the water partings within them may well have 

 been a thousand feet higher. 



One of the more important passages of the Alexander 



